Oinker's pen (3)
I believe the answer is:
sty
'oinker's pen' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sty that I've seen before include "Piggery" , "Pig home or eye sore" , "Enclosure for swine" , "Animal pen" , "Porcine pen" .)